ATRP is considered to be one of the most successful controlled/“living” radical processes (CRP) and has been thoroughly described in a series of co-assigned U.S. patents and applications, such as U.S. Pat. Nos. 5,763,548; 5,807,937; 5,789,487; 5,945,491; 6,111,022; 6,121,371; 6,124,411; 6,162,882; 6,407,187; 6,512,060; 6,538,091; 6,541,580; 6,624,262; 6,624,263; 6,627,314; 6,759,491; and U.S. patent application Ser. Nos. 09/534,827; 09/972,056; 10/034,908; 10/269,556; 10/289,545; 10/638,584; 10/860,807; 10/684,137; 10/781,061 and 10/992,249 all of which are herein incorporated by reference. ATRP has also been discussed in numerous publications with Matyjaszewski as co-author and reviewed in several book chapters. [ACS Symp. Ser., 1998, 685; ACS Symp. Ser., 2000; 768; Chem. Rev. 2001, 101, 2921-2990; ACS Symp. Ser., 2003; 854.] Within these publications, similar polymerizations may be referred to by different names, such as transition metal mediated polymerization or atom transfer polymerization, but the processes are similar and referred to herein as “ATRP”.
A controlled radical polymerization (“CRP”) process is a process performed under controlled polymerization conditions with chain growth proceeding via a radical mechanism, such as, but not limited to, ATRP stable free radical polymerization, (“SFRP”) most frequently, nitroxide mediated polymerization, (“NMP”) reversible addition-fragmentation transfer, (“RAFT”) or degenerative transfer systems. A feature of CRP is the creation of an equilibrium between active polymer chain and dormant polymer chain. In certain embodiments, it may be preferable if a majority of polymer chains are present as dormant polymer chains. The equilibrium between the active and dormant chains typically provides for more controlled chain growth relative to conventional radical polymerization. CRP processes are capable of producing mere uniform polymers; however, the active propagating chain may react in termination reactions resulting in higher polydispersities. Therefore, typically, to minimize termination reactions, the instantaneous concentration of active propagating species is maintained at a low concentration.
In CRP, the ability to maintain or adjust the equilibrium between active and dormant species and quantitative initiation early in the polymerization process allows, under appropriate conditions, the capability for synthesis of polymers with special architecture and functionality. In addition, if desired, the overall rate of monomer conversion may occur at rates equivalent to uncontrolled polymerization. Controlled polymerization process may be used to prepare polymers having a degree of polymerization that may be approximated from the ratio of the amount of consumed monomer to the initiator, a polydispersity close to a Poisson distribution and functionalized chain ends.
As used herein, “polymer” refers to a macromolecule formed by the chemical union of monomers, typically five or more monomers. The term polymer includes homopolymer and copolymer block copolymers, and polymers of any topology including star polymers, block copolymers, gradient copolymers, periodic copolymers, telechelic polymers, bottle-brush copolymers, random copolymers, statistical copolymers, alternating copolymers, graft polymers, branched or hyperbranched polymers, comb polymers, such polymers tethered from particle surfaces, as well as other polymer structures.
ATRP is the most often used CRP technique with a significant commercial potential for many specialty materials including coatings, sealants, adhesives, dispersants but also materials for health and beauty products, electronics and biomedical applications. The most frequently used ATRP is based on a simple reversible halogen atom transfer catalyzed by redox active transition metal compounds.
Certain advantages of an ATRP are as follows, many commercially available initiators may be used and various macroinitiators, including wafers, colloids, glass, paper, and bio-active molecules including proteins, DNA, carbohydrates and many commercial polymers may be simply synthesized; many polymers produced by ATRP allow facile functionalization or transformation of the end groups by replacing terminal halogens with azides, amines, phosphines and other functionalities via nucleophilic substitution, radical addition or other radical combination reactions; an abundance of polymerizable monomers are available; allows production of macromolecules with complex topology such as stars, combs and dendrimers, coupled with the ability to control composition (block, gradient, periodic copolymers) and even control of polymer tacticity; and allows a simple reaction which may be carried out in bulk, or in the presence of organic solvents or in water under homogeneous or heterogeneous conditions, in ionic liquids, and CO2.
However, in certain applications, concentration of the transition metal catalyst in an ATRP polymerization medium may have to be reduced in the final product. As such, there have been several methods developed to remove or reduce the amount of transition metals in the process, but these add additional cost to the preparation of polymers by ATRP. The methods used to reduce the concentration of catalysts in the final product include development of more active catalysts, for example CuBr complexed by Me6TREN is ˜10,000 more active than catalysts complexed by bipyridine ligands; catalysts have been immobilized on solids; hybrid catalyst systems comprising both immobilized and small concentrations of soluble catalysts (˜10-20 ppm). There are also several methods developed to recover and regenerate catalysts, including separating the catalyst by filtration, precipitation or extraction. For example, CuBr/PMDETA complex may be oxidized to Cu(II) species by expose to air and quantitatively extracted from toluene to water, resulting, in some cases, with as little as <1 ppm of catalyst remaining in the polymer. There is therefore a need to reduce the concentration of catalyst while maintaining polymer reaction rate and retaining control over MW and PDI and there exists a need for more efficient methods to reduce the catalyst concentration in polymers produced by ATRP.
Three different ATRP initiation methods, or activation reactions, have been disclosed: normal ATRP initiation, “reverse” ATRP initiation, and simultaneous normal and reverse initiation (SR&NI) ATRP. See U.S. Pat. Nos. 5,763,548 and 6,759,491.
Typically, ATRP processes comprise a transition metal complex. The transition metal complex may participate in a repetitive redox reaction homolytically removing a radically transferable atom or group from an initiator molecule or dormant polymer chain, Pn—, to form the active propagating species, P*n, and then deactivating active propagating species, P*n, by donating back a transferable atom or group. (Scheme 1)

The transition metal catalyst for this repetitive addition process must be present, at least partially, in the lower oxidation state, or activator state, Mtn/Ligand. However, typically, the lower oxidation state of the transition metal catalyst is readily oxidized. Therefore, there are inherent difficulties in handling the catalyst associated with large scale bulk and solution based polymerization processes and in emulsion and mini-emulsion processes where trace levels of oxygen should be removed. The typical ratio of activator (Mtn/Ligand) to deactivator (X-Mtn+1/Ligand) varies with the specific monomers and the polarity of the reaction medium, as well as other factors, between 99 parts activator to 1 part deactivator to 5 parts activator to 95 parts deactivator.
Any transition metal complex capable of maintaining the dynamic equilibrium and participate in a redox reaction comprising the transferable atom or group with the polymer chain may be used as the catalyst in ATRP, and many examples are discussed in the cited art. A suitable equilibrium can be formed after consideration of oxidation states, complex formation with suitable ligands and redox potential of the resulting complex to provide a catalyst for the desired (co)polymerization of a wide range of comonomers. A wide variety of ligands have been developed to prepare transition metal catalyst complexes that display differing solubility, stability and activity.
Normal ATRP Initiation
Typically, ATRP processes are initiated by the redox reaction between an initiator comprising one or more transferable atom(s) or group(s) and a catalyst complex comprising a transition metal salt in a lower oxidation state complexed with a ligand, solvent molecule or monomer. The transferable atom or group is an atom or group that may be homolytically cleaved from the initiator by the catalyst, thereby oxidizing the catalyst to a higher oxidation state and forming an active propagating species capable of monomer addition. After initiation, an ATRP process, generally, is based on a dynamic equilibrium between a transition metal complex reversibly activating and deactivating the polymer chain via a similar homolytic atom or group transfer via a redox reaction. (Scheme 1) During the dynamic equilibrium the transition metal complex cycles between a lower oxidation state and a higher oxidation state.
The advantages of normal initiation of ATRP include that the added initiator molecule includes the transferable atom or group needed to initiate and subsequently repeatedly terminate each polymer chain, therefore no additional transferable atoms or groups are required to be added by other components of the polymerization process. Therefore, adding sufficient transition metal complex in the lower oxidation state provides suitable catalytic activity to the process. By “suitable catalytic activity” it is meant that the polymerization comprises an amount of catalyst needed to drive the reaction to a desired degree of polymerization with appropriate heat control to produce a polymer with the desired properties. Typically, an ATRP process requires a sufficient catalyst amount to compensate for any loss of catalytic activity due to termination reactions.
ATRP catalysts may vary in catalytic activity based upon the properties of the transition metal, the ligands and the temperature and polarity of the reaction medium, as well as other factors. Generally, more active catalysts are less oxidatively stable in their lower oxidation states. Due to this oxidative instability, active catalysts in their lower oxidation states are more difficult to handle; for instance, trace levels of oxygen or other oxidants should be to be removed from the polymerization medium prior to addition of the active catalyst in a lower oxidation state to prevent the catalyst from being converted to the higher oxidation state deactivator.
Reverse ATRP Initiation
In a reverse ATRP, a more stable catalyst complex in the higher oxidation state may be added to the polymerization medium. Generally, the higher oxidation state of a transition metal complex is a lower cost and more oxidatively stable state of the complex and may often be stored in the presence of air.
In reverse ATRP, as opposed to normal ATRP, the transferable atom or group begins as a counterion or ligand on the transition metal salt or transition metal complex in the higher oxidation state. A “reverse ATRP” the reaction is then initiated by generation of a radical by known processes, such as by decomposition of a standard free radical initiator which either directly participates in a redox reaction with the higher oxidation state transition metal forming the transition metal complex in the lower oxidation state, and a molecule with a transferable atom suitable for initiation of an ATRP reaction, or it may initiate a polymerization that is quickly deactivated by the transition metal complex in the higher oxidation state. Typically, reverse ATRP processes require a high catalyst concentration in order to introduce the appropriate concentration of radically transferable atoms or groups to the reaction to both maintain a controlled polymerization and attain polymers of the desired molecular weight at high conversion of monomer to polymer.
In addition, a typical reverse ATRP process must be initiated in a narrow temperature range to ensure efficient thermal decomposition of the standard free radical initiator to reduce the catalyst complex and produce polymers with low polydispersities. Further, since the first radicals are provided by normal radical initiators, it is not as easy to prepare homo-telechelic polymers, block, or graft copolymers of more complex architecture than with normal initiation.
SR&NI ATRP
A SR&NI polymerization process comprises a dual initiation system for atom transfer radical polymerization. The initiation system comprises aspects of both standard free radical initiators and initiators comprising a transferable atom or group. The dual initiation system may be used to prepare any type of polymer that may be prepared by ATRP, such as, but not limited to, homopolymers, random, statistical, gradient, alternating copolymers, block, graft, branched or hyperbranched, star, comb, and bottle brush as well as other polymer structures.
However, polymerization in an SR&NI polymerization proceeds from two different initiators. In certain embodiments, this may be desirable. For example, if one initiator is a macroinitiator used to form a block copolymer in the “normal” initiated ATRP, but the conventional radical initiator added to form the active catalyst complex in a “reverse ATRP” will form a homopolymer that may be considered an undesirable byproduct for certain applications.

Thus, there is a need for an improved ATRP process that avoids such limitations.